Susimail is a web-based e-mail client for the I2P network.
I2P has a free pseudonymous e-mail service run by an individual called 'Postman'. The mail transfer servers are pop.mail.i2p (POP3) and smtp.mail.i2p (SMTP).
Susimail was created to address privacy concerns in using these servers directly using traditional email clients, such as leaking the user's hostname while communicating with the SMTP server. Susimail is a web-based email client intended primarily for use with Postman's mail servers, and is designed with security and anonymity in mind. It is currently included in the default I2P distribution, and can be accessed through the I2P router console web interface. (Note that this is only used to read and send e-mail, not to create or manage your mail.i2p account; the latter must be done at hq.postman.i2p.)